Shallow contamination of soil, and of paved surfaces, by persistent chemicals is a widespread problem arising from many human activities. Frequently, chemical compounds are spilled or placed on the surface of soils for dust or weed control, or for other agricultural purposes. Under these circumstances, the materials are often spread over very large areas. If the chemicals are judged to be detrimental to human health or need to be dealt with for other reasons, the task of removing them or remediating the site is formidable. If the contaminating chemicals are sufficiently insoluble in water, they will not be leached into the soil by downward percolating rainwater, but will remain very near the surface for many years. Even with agricultural activities, such as ploughing, discing, and harrowing, these contaminants will seldom be carried as deep as a foot and usually will remain in the upper few inches of the soil.
The most direct method of remediating a shallow, contaminated site is to simply remove the soil by digging it up and carrying it to a disposal site. Disturbance in the soil by excavation produces dispersion and secondary contamination. When dry, this occurs by windblown dust, and when wet, by rutting and other deep mixing of the soil. Because excavation with large equipment can spread the contamination into the soil, additional clean soil must be taken to assure complete removal of the contaminants. Furthermore, the transport of the offending chemicals to another location does not solve the overall problem because the new site becomes a potential problem and a continuing liability to the original operators.
Some other previously employed methods address the necessity of destroying the contaminants; however, most of these methods also require: (1) excavation, (2) transport to a treatment facility, (3) some other means of destruction or removal from the soil, and (4) finally return of the soil to the original location. All of these methods suffer the hazards of secondary contamination from dusty excavations and process plant operations, such as incineration. This is of particular concern if there is nearby habitation.
Other previously employed methods include bioremediation or treatment of the soils by some kind of chemical washing. These methods are not widely used because they have not proven to be sufficiently effective in removing contaminates to the extremely low levels of residual concentration generally required. This is particularly a problem if the contaminates are highly toxic or carcinogenic.
U.S. Pat. No. 4,984,594 discloses a method whereby the soil is heated in-situ to remove contaminants. Volatile substances in the soil are vaporized by the heat and drawn upward and gathered into a vacuum system where they are condensed in traps for subsequent treatment (e.g. incineration), or disposal. Soil heating is effected by means of an areal electric heater that is placed upon the soil surface. The heater, which is a mesh of electrically-conductive wires, is in good contact with the soil. Heat is transferred into the soil by thermal conduction. A permeable layer may be placed above the heater if the wire mesh does not provide a sufficient conduit for horizontal transport of gases to a central vacuum port. A layer of high temperature insulation is placed above the heater/permeable layer, and above that is placed a flexible, but impermeable silicone rubber sheet. A vacuum port in the impermeable sheet is connected to the collection/condensation system or to an incinerator. Another, more durable, heater is disclosed in U.S. Pat. No. 5,229,583. This heater consists of a metallic furnace belt with tubular electric heating elements inserted between the rungs of the belt. Compared to the previous design, the furnace belt (1) allows the heater to be rolled or dragged from one location to another, (2) provides a highly permeable path for vapor flow, and (3) distributes heat laterally between the heating elements.
Another heater blanket design, disclosed in U.S. Pat. No. 5,221,827, incorporates a ceramic beaded conductive wire heater that is "floatingly pinned to a layer of ceramic fiber insulation." This design includes two additional layers of insulation, a rigid stainless steel support frame, and an impermeable canopy over the support frame over the heater.
Any of the heater blankets for in-situ remediation that are cited above are capable of removing contaminants from a small, flat, test site, such as the one described in U.S. Pat. No. 5,229,583. In practice, however, soil surfaces are often uneven so that heat transfer is poor wherever the heater does not touch the soil surface. Because soil is a relatively poor thermal conductor, the spread of heat laterally is also slow. This limits the rate at which heat can be transferred to the soil without overheating the mineral insulated heating elements, and requires closely spaced heating elements, resulting in increased costs. Another problem of the prior art heaters is that of overheating of the silicone rubber vacuum sheet. This is a troublesome operational hazard that will reduce the temperature at which the surface heater can be reliably operated. Still another problem is that the insulating layer between the heater and the vacuum sheet tends to be compressed by atmospheric pressure, thereby reducing its insulating capacity. Yet another disadvantage is that the large quantities of water vapor emanating from the soil tend to condense in the insulation, particularly around the circumference, resulting in a soggy insulation which may contain contaminants. For large scale applications, both mechanical and economic constraints limit the use of the prior art designs.
It is therefore an object of the present invention to provide a heater and a method to use such heater that is less expensive and will also conserve electrical power through increased thermal efficiency. It is another object to provide such an apparatus and method that can be easily moved from location to location.